1. Field of the Invention
The present invention relates to motors, and more particularly to a disk driving motor built in a disk drive apparatus.
The height of disk drive apparatuses such as a magnetic disk drive apparatus or an optical disk drive apparatus has been reduced. A brushless dc motor of an axial-gap type is particularly suitable for the purpose of reducing the height of a disk drive and is used extensively as a disk driving motor.
With the increase in the recording density, there is a requirement for a stable control of the rotational speed of the disk driving motor.
FIG. 1 is a block diagram of a driving control system of a disk driving motor 10. The disk driving motor 10 has a stator part 11 and a rotor part 12, and is built in a disk drive apparatus 20.
The rotor part 12 is opposite to the stator part 11 via an axial gap 13.
The rotor part 12 has a disc-shaped rotor magnet 14 magnetized so that multiple magnetic poles are disposed in the circumferential direction.
The stator part 11 has a driving coil composite 15 which is annularly arranged and a frequency generation pattern 16 which is also annularly arranged.
A motor driving circuit 17 and a servo system circuit 18 are also provided for the motor 10.
The servo system circuit 18 generates a servo signal as a result of comparing a rotational speed detecting signal from the frequency generation pattern 16 and regenerated clocks from a PLL.
The motor driving circuit 17 outputs a motor driving signal for generating a rotating magnetic field around the driving coil composite 15. The rotating magnetic field causes the rotor part 12 to rotate.
While being driven, the motor 10 (rotor part 12) has its rotational speed controlled in accordance with the rotational speed detecting signal and the regenerated clocks so that a disk (not shown) is rotated at a constant angular or linear speed.
Accordingly, in order to read information from and write information to the disk in a satisfactory manner in terms of the quality, it is necessary for the rotational speed control of the motor 10 to be performed with precision. For this purpose, it is essential that the rotational speed detecting signal characterized by no degradation in the S/N ratio be obtained.
The arrangement of the driving coil composite 15 and the frequency generation pattern 16 in the stator part 11 will now be examined closely.
In one conceivable arrangement of a stator part 11A shown in FIG. 2A, the annular driving coil composite 15 is disposed toward the periphery and the annular frequency generation pattern 16 is disposed toward the center. In another conceivable arrangement of a stator part 11B shown in FIG. 2B, the frequency generation pattern 16 is disposed toward the periphery and the driving coil composite 15 is disposed toward the center.
It is known that the former arrangement provides a larger output torque of the motor, a smaller power consumption and a greater driving efficiency than the latter.
Therefore, the motor 10 is usually constructed to have the stator part 11A as shown in FIG. 2A.
However, it is to be noted that two lead lines 25 and 26 for leading the rotational speed detecting signal outside the driving coil composite 15 are provided in the motor 10 of the above construction so as to traverse the driving coil composite 15 in a top view.
For this reason, the lead lines 25 and 26 are subject to the influence of the rotating magnetic field generated by the driving coil composite 15 and the influence of the magnetic field of the rotor magnet 14. Thus, it is likely that an unnecessary current (noise) is induced.
Further, it is difficult to obtain a high-level rotational detecting signal according to the construction of FIG. 2A.
If a noise is generated in the lead lines 25 and 26, the S/N ratio of the rotational speed detecting signal easily drops.
A drop in the S/N ratio of the rotational speed detecting signal may invite an unstable control of the rotational speed.
Accordingly, a scheme for preventing noise in the lead lines 25 and 26 as much as possible is required.
2. Description of the Related Art
FIGS. 3A and 3B show a three-phase brushless dc motor 30 of an axial-gap type. FIG. 3A shows a rotor magnet 31 of the motor 30 and FIG. 3B shows a stator part 40 of the motor 30.
The rotor magnet 31 is disc-shaped and magnetized so that there are twenty magnetic poles including N-poles 33N and S-poles 33S disposed in the circumferential direction. The adjacent N-pole and S-pole form a separation angle .alpha..
The stator part 40 comprises a substrate 41 and a driving coil composite 42 fixed to the substrate 41.
The substrate 41 has a frequency generation pattern 43, and lead lines 44 and 45 provided on the surface of the pattern 43.
The driving coil composite 42 is constructed such that three types of phase coils are periodically arranged in the circumferential direction. Specifically, a U-phase coil 42U, a V-phase coil 42V, a W-phase coil 42W, a U-phase coil 42U, a V-phase coil 42V . . . are arranged in the stated order. The total number of coils (magnetic poles) is fifteen.
When the motor 30 is driven, the U-phase coil 42U, the V-phase coil 42V, the W-phase coil 42W are energized in the stated order so that the U-phase coil 42U, the V-phase coil 42V, the W-phase coil 42W . . . are excited in the stated order. As a result, a rotating magnetic field is produced so that the rotor magnet 31 is rotated.
While being rotated, the rotor magnet 31 produces an alternating magnetic flux.
The lead lines 44 and 45 extend straight in respective radial directions with a separation of an angle .alpha. (equal to the angle .alpha. between the N-pole and the S-pole).
Since the lead lines 44 and 45 are separated by the angle .alpha., the alternating magnetic flux produced by the rotation of the rotor magnet 31 is canceled by the magnetic flux in a one-turn coil formed by the lead lines 44 and 45. Therefore, no current is induced in the one-turn coil. That is, no current is produced in the lead lines 44 and 45.
Accordingly, a rotational speed detecting signal carrying no noise is obtained at terminals 46 and 47.
The conventional motor 30 described above has the following two problems (i) and (ii).
(i) A current may be induced as a result of a variation in precision with which the motor is assembled.
Ampere's law provides the following relationship: EQU .phi..sub.c B.multidot.dS=.mu..sub.0 I (1)
where S indicates an area formed by any given closed curve (circuit) C, B a density of magnetic flux passing through the area, and I a current induced in the closed curve.
In the conventional motor 30, the current I as defined by the equation (1) provides a noise.
The equation (1) tells us that noise reduction can be achieved by reducing the magnetic flux B.multidot.dS, that is, by reducing the magnetic flux density B.
Since the lead lines 44 and 45 extend straight in the radial directions of the motor, the one-turn coil formed by the lead lines 44 and 45 is subject to the effect of a variation in precision with which the magnetic poles 33N and 33S of the rotor magnet 31 are positioned or to the effect of an eccentricity of the rotor magnet 31.
As a result of the above-mentioned effects, the magnitude of the volume of the magnetic flux in the N direction differs from that of the magnetic flux in the S direction, thus causing a net magnetic flux to remain. Because of this net magnetic flux that remains, a noise is produced so that the S/N ratio of the rotational speed detecting signal is degraded.
The degradation in the S/N ratio becomes a problem in mass-producing the motor.
(ii) A noise is produced due to a rotating magnetic field produced by the driving coil composite 42.
Referring to FIG. 3B, the lead line 44 passes under the V-phase coil 42W and the lead line 45 passes under the W-phase coil 42V, the U-phase coil 42U residing between the lead lines 44 and 45.
An examination will be given below of the ratio between areas by which a band-like area 55 provided between the lead lines 44 and 45 overlaps each of the U-phase coil 42U, the V-phase coil 42V and the W-phase coil 42W.
The entirety of an area Su of the U-phase coil 42U overlaps the area 55.
A 1/3 of an area Sv (about 0.3 Sv) of the V-phase coil 42V overlaps the area 55.
A 1/5 of an area Sw (about 0.2 Sw) of the W-phase coil 42W overlaps the area 55.
The areas of the phase coils 42U, 42V and 42W are identical to each other. That is, Su=Sv=Sw.
The ratio between areas by which the area 55 overlaps each of the coils 42U, 42V and 42W is given by: EQU Su:0.3Sv:0.2Sv=1.0:0.3:0.2
The applicants of the present invention carried out an experiment in which the paths of the lead lines 44 and 45 are varied in order to analyze the associated density of the magnetic flux of the rotating magnetic field produced by the driving coil composite 42 and passing through the area 55 between the lead lines 44 and 45.
The analysis is based on the fact that the density B of the magnetic flux produced by the driving coil composite 42 and passing through the area 55 between the lead lines 44 and 45 underneath the driving coil composite 42 is given by: EQU B(t)=Bu(t)+Bv(t)+Bw(t).apprxeq.0
where Bu is a density of the magnetic flux produced by the U-phase coil 42U, Bv a density of the magnetic flux produced by the V-phase coil 42V, Bw a density of the magnetic flux 42W produced by the W-phase coil 42W and t a time.
FIG. 4 shows a result obtained through the experiment.
FIG. 4 is a table showing relative values of the density of the magnetic flux passing through the area 55, obtained when the area of the U-phase coil 42u is fixed to a relative value of 1.0, and the area of the V-phase coil 42V and the area of the W-phase coil 42W are varied. More specifically, the experiment was carried out such that the area 55 between the lead lines 44 and 45 always includes the entirety of the U-phase coil 42U, and such that the areas of the V-phase coil 42V and the W-phase coil 42W included in the area 55 are varied.
Referring to FIG. 4, 1 indicates the magnetic flux density existing in the area 55 when the area 55 includes only the entirety of the U-phase coil 42U.
2 indicates a case where the area 55 includes the entirety of the U-phase coil 42U and the entirety of the V-phase coil 42V.
3 indicates a case where the area 55 includes the entirety of the U-phase coil 42U and the entirety of the W-phase coil 42W.
4 indicates a case where the area 55 is made to lie as shown in FIG. 3B.
In the above-described cases 1, 2 and 3, the magnetic density has a maximum relative value of 1.00.
It will be noted that the lead lines 44 and 45 are subject to the influence of the rotating magnetic field produced by the driving coil composite 42 to a largest degree when the paths of the lead lines 44 and 45 are set as in the cases 1, 2 and 3 described above. As a result, noise arising from the induced current is at a maximum level in these cases.
The relative level of the noise in these cases is designated as 1.000.
In the conventional example shown in FIG. 3B, the density of the magnetic flux passing through the area 55 has a relative value of 0.755, which is comparatively high.
Therefore, in the motor 30 shown in FIGS. 3A and 3B, a noise having a relatively high level of 0.755 is produced due to the rotating magnetic field produced by the driving coil composite 42.
Consequently, it is inevitable that the S/N ratio of the rotational speed detecting signal is degraded.
To summarize the above, the conventional motor 30 has a problem in that the S/N ratio of the rotational speed detecting signal is easily degraded due to a variation in precision with which the motor is assembled. A further problem is that, even if precision of the assembly is improved, the rotating magnetic field produced by the driving coil composite 42 is bound to invite a degradation in the S/N ratio of the rotational speed detecting signal.